The present invention is applicable to any type of known matrix screen which functions via transmission or emission and more particularly to passive multiplexed liquid crystal screens making it possible to display on each pixel one of two states to be denoted for the sake of convenience by one black state and one white state.
It is also known that the addressing of matrix screens comprising K lines and M columns is effected via the successive scanning of different lines. For a time equal to the line time Tl, one line l is "open" by applying a voltage VL. During this period, the M image data signals are addressed onto the M columns in the form of voltages, for example in the case of liquid crystals .+-.VC. The pixel situated at the intersection of the line l and the column m is thus excited by a potential difference VL.+-.VC which, depending on its location with respect to the switching threshold of the liquid crystal of the screen, renders it transparent (white state) or opaque (black state).
Such screens are well known and are described in detail in the documents EP-A-0 092 181 and EP-A-0 032 362.
More specifically, the present invention concerns the problem of writing on such screens a certain number Q of shades or levels of grey Gj on a black and white or colored image.
The obtaining of a shade of grey Gj on a specific pixel may up until now only result in the display on this pixel of a sequence comprising a certain number of white and black states, the perception of the resultant grey being clearly proportional to the ratio of the number of these states of one category and of the other; this perception may result in either an interpolation due to the functioning of the device itself (in the case of liquid crystals) or in the visual phenomenon which makes use of the persistance of retinal impressions. The levels of grey Gj thus defined are generally speaking "equidistant", that is, it is possible to move from one to the other discontinuously by adding or subtracting a black state or white state in the display sequences. So as to inscribe levels of greys, it is necessary to superimpose several black and white states on each pixel, which involves scanning the screen several times during successive sub-frames and relating to a given image. As for the frames, this scanning takes place line by line. The state of the art is this respect is clearly demonstrated by the document entitled "Conference Records of the 1985 International Display Research Conference, San Diego, Calif., 15-17 Oct. 1985, pages 213 to 220", as well as by the documents EP-A-0 219 479 and FR-A-88 12 810 of 30 Sep. 1988.
All the methods of the prior art briefly mentioned above making it possible to display such shades of greys do nevertheless have to various degrees a certain number of drawbacks not fully resolved to date and which may be denoted under the term of "coupling" signifying in fact that the displaying of a specific pixel of such a screen is not completely independent of the displaying of adjacent pixels. These drawbacks are well-known to experts in this field and are described in detail in the document relating to "Proceedings of the SID Conference" held at Las Vegas in May 1990, pages 412 to 415.
First of all, a coupling exists between the various pixels of a given column for the following reason: when a specific pixel of this column is excited, the data signal applied in the form of voltage steps .+-.VC to the column in question arrives at the same time at all the other pixels of this column which do not correspond to lines selected at this moment. In other words, this results in obtaining stray signals which disturb the instantaneous state of these other pixels. More precisely, the coupling between the pixels of a given column mainly depends on the number of black to white transitions present in the column in question. In other words, this drawback is that much more marked when the number of passages from black to white in the pixels of a given column is high.
This defect is a defect inherent in the addressing principle of the screen itself. Coupling depends on the image displayed, which is totally random as regards the successions of white and black pixels. In fact, in all the grey display methods of the prior art mentioned earlier, the scanning method randomly brings about successions of white and blacks to the various pixels according to the various image addressing phases. Accordingly, this results in a temporal correlation or interaction between the method for scanning the screen and the image needed to be displayed on it, which thus may randomly accentuate the actual defects of the screen without offering any possible recourse.